U.S. patent number 10,058,408 [Application Number 15/461,422] was granted by the patent office on 2018-08-28 for personal hygiene device with resonant motor.
This patent grant is currently assigned to The Procter & Gamble Company. The grantee listed for this patent is Braun GmbH. Invention is credited to Torsten Klemm, Kervin Kuchler, Andreas Moehring, Norbert Schaefer, Martin Stratmann, Carl Stuckrath.
United States Patent |
10,058,408 |
Klemm , et al. |
August 28, 2018 |
Personal hygiene device with resonant motor
Abstract
A personal hygiene device has a resonant motor and a motor
control unit for applying a periodic voltage signal with a driving
frequency at the resonant motor for driving the resonant motor into
an oscillating motion with an oscillating frequency equal to the
driving frequency. The motor control unit comprises a synthesizer
circuit for digitally synthesizing the periodic voltage signal from
voltage pulses of variable length provided with a pulse frequency
higher than the driving frequency such that at least two voltage
pulses are applied at least in one of two half cycles of each
period of the periodic voltage signal.
Inventors: |
Klemm; Torsten (Frankfurt,
DE), Kuchler; Kervin (Darmstadt, DE),
Moehring; Andreas (Kronberg, DE), Schaefer;
Norbert (Frankfurt, DE), Stratmann; Martin (Bad
Soden, DE), Stuckrath; Carl (Friedberg,
DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Braun GmbH |
Kronberg |
N/A |
DE |
|
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Assignee: |
The Procter & Gamble
Company (Cincinnati, OH)
|
Family
ID: |
50982818 |
Appl.
No.: |
15/461,422 |
Filed: |
March 16, 2017 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20170189150 A1 |
Jul 6, 2017 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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14749557 |
Jun 24, 2015 |
9628014 |
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Foreign Application Priority Data
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Jun 26, 2014 [EP] |
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14174206 |
May 27, 2015 [EP] |
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15169330 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A45D
26/00 (20130101); A61C 15/047 (20130101); A61C
17/34 (20130101); H02P 25/032 (20160201); A61C
17/20 (20130101); A61C 17/221 (20130101); H02P
27/08 (20130101); A61C 17/3481 (20130101); A61H
7/00 (20130101); A45D 2026/008 (20130101); A61B
17/244 (20130101) |
Current International
Class: |
H02K
33/00 (20060101); A61C 17/22 (20060101); A45D
26/00 (20060101); A61C 17/34 (20060101); H02P
25/032 (20160101); H02P 27/08 (20060101); A61C
15/04 (20060101); A61H 7/00 (20060101); A61B
17/24 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Masih; Karen
Attorney, Agent or Firm: McCrary; Parker D. Vladimir
Vitenberg
Claims
What is claimed is:
1. A personal hygiene device comprising a resonant motor; a motor
control unit for applying a periodic voltage signal with a driving
frequency at the resonant motor for driving the resonant motor into
an oscillating motion with an oscillating frequency equal to the
driving frequency; wherein the motor control unit comprises a
synthesizer circuit for digitally synthesizing the periodic voltage
signal from voltage pulses of variable length provided with a pulse
frequency higher than the driving frequency such that at least two
voltage pulses are applied at least in one of two half cycles of
each period of the periodic voltage signal, wherein the
motor-control unit is arranged to measure a back EMF at the motor
coil when essentially no motor current flows therethrough.
2. The personal hygiene device in accordance with claim 1, further
comprising a user controllable input unit for influencing the
periodic voltage signal applied by the motor control unit, in
particular for influencing the shape of the periodic voltage
signal.
3. The personal hygiene device in accordance with claim 2, wherein
the user controllable input unit comprises a control element for
selectively influencing the periodic voltage signal, in particular
wherein the control element is arranged at a handle section of the
personal hygiene device.
4. The personal hygiene device in accordance with claim 2, wherein
the user controllable input unit comprises a separate control
device that is physically separate from a handle section of the
personal hygiene device and the personal hygiene device further
comprises a wireless connection unit for establishing a wireless
connection between the handle section and the separate control
device.
5. The personal hygiene device in accordance with claim 4, wherein
the separate control device, in particular a smart phone or a
tablet computer, has a control element for selectively influencing
the periodic voltage signal, in particular the shape of the
periodic voltage signal.
6. The personal hygiene device in accordance with claim 1, wherein
the synthesizer circuit comprises a memory unit in which at least
one look-up table of voltage pulse length values for at least one
half-cycle of the periodic voltage signal is stored.
7. The personal hygiene device in accordance with claim 2, wherein
the synthesizer circuit comprises a memory unit in which at least
two look-up tables that each comprise voltage pulse length values
for at least one half-cycle of the periodic voltage signal are
stored and the user controllable input unit is arranged to
influence which look-up table is used to generate the periodic
voltage signal.
8. The personal hygiene device in accordance with claim 1, wherein
the pulse frequency of the synthesizer circuit is at least 6 times
as high as the driving frequency, optionally wherein the pulse
frequency of the synthesizer circuit is at least 20 times as high
as the driving frequency and further optionally wherein the pulse
frequency is at least 100 times as high as the driving
frequency.
9. The personal hygiene device in accordance with claim 1, wherein
the pulse frequency is above 18 kHz, optionally wherein the pulse
frequency is above about 20 kHz and further optionally below 100
kHz.
10. The personal hygiene device in accordance with any one of claim
1, wherein a clocking frequency of the synthesizer circuit is at
least 32 times the pulse frequency, optionally at least 128 times
the pulse frequency, and further optionally at least 256 times the
pulse frequency.
11. The personal hygiene device in accordance with claim 1, wherein
the periodic voltage signal is one of a sine wave signal, a
triangle signal, a trapeze signal, or a saw-tooth signal.
12. The personal hygiene device in accordance with claim 1, wherein
the motor control unit comprises a digital voltage circuit for
providing a single voltage pulse per half cycle at the resonant
motor.
13. The personal hygiene device in accordance with claim 12,
wherein the motor control unit is arranged to compose the periodic
voltage signal by selectively switching between the synthesizer
circuit and the digital voltage circuit, and wherein said selective
switching happens at least once during at least one half cycle per
period of the periodic voltage signal.
14. The personal care device of claim 1, wherein the motor control
unit is arranged to switch off a driving signal during at least one
of the half cycles of each period.
15. The personal care device of claim 14, wherein the motor control
unit is arranged to cause the driving signal to stay at zero until
a measurement of the back EMF has been made.
16. The personal care device of claim 14, wherein the motor control
unit is arranged to short-circuit the motor to achieve a fast
current discharge.
17. The personal care device of claim 15, wherein the motor control
unit is arranged to have, after the measurement of the back EMF has
been made, the voltage pulses resumed in the same half cycle in
which the voltage pulses were stopped.
18. The personal care device of claim 15, wherein the motor control
unit is arranged to cause the voltage pulsed switched off in a
complete second quadrant of the half cycle in which the voltage is
switched off.
Description
FIELD OF THE INVENTION
The present invention is concerned with a personal hygiene device
having a resonant motor that is driven by a motor control unit into
an oscillating motion.
BACKGROUND OF THE INVENTION
It is known that a resonant motor (i.e. a motor that can
essentially be described as a spring-mass system having a resonant
behavior such that the motor is particularly efficient when driven
at or closely around its resonance frequency) can be driven into an
oscillating motion by periodically applying an essentially
rectangular voltage signal in every half cycle of a period of the
oscillating motion, where the voltage signal is applied with
alternating sign in the different half cycles of an individual
period. The resonant motor may be arranged in the bridge section of
an H-bridge circuit by which the applied voltage signal can be
commuted, i.e. inverted, and motor current can be discharged from
the motor coil prior to the change of the motion direction.
Document WO 2004/034561 A1 generally discusses a resonant motor
arranged in a bridge section of a H-bridge circuit and a driving
scheme by applying rectangular voltage pulse signals at the motor
by a motor control unit comprising the H-bridge circuit.
It is further known that resonant motors can be used in personal
hygiene devices such as electric toothbrushes or electric
shavers.
It is an object of the present disclosure to provide a personal
hygiene device with a resonant motor that is improved over the
known personal hygiene devices, in particular with respect to its
noise behavior.
SUMMARY OF THE INVENTION
In accordance with one aspect there is provided a personal hygiene
device comprising a resonant motor, a motor control unit for
applying a periodic voltage signal with a driving frequency at the
resonant motor for driving the resonant motor into an oscillating
motion with an oscillating frequency equal to the driving
frequency, wherein the motor control unit comprises a synthesizer
circuit for digitally synthesizing the periodic voltage signal from
voltage pulses of variable length provided with a pulse frequency
higher than the driving frequency such that at least two voltage
pulses are applied at least in one of two half cycles of each
period of the periodic voltage signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The proposed personal hygiene device will be further elucidated by
a detailed description of example embodiments and by reference to
figures. In the figures
FIG. 1 is a schematic depiction of an example embodiment of a
personal hygiene device in accordance with the present
disclosure;
FIG. 2 is a schematic depiction of a motor control unit for driving
a resonant motor;
FIG. 3 is a schematic depiction of a motor control unit comprising
a synthesizer circuit for digitally synthesizing a periodic voltage
signal for driving a resonant motor; and
FIG. 4 is a schematic depiction of a half-cycle of a sinusoidal
periodic voltage signal composed from voltage pulses of variable
length.
DETAILED DESCRIPTION OF THE INVENTION
A "resonant motor" (or oscillating motor) in accordance with the
present disclosure means a motor that has a resonant oscillation
behavior. A resonant motor can be mathematically expressed as a
harmonic oscillator, i.e. a spring-mass system. A resonant motor in
accordance with the present disclosure is driven into oscillating
motion by periodic application of an external force, in particular
a periodic voltage signal as will be explained in the following
paragraphs. The amplitude of the moving part of the resonant motor
becomes maximal when the driving frequency of the external driving
force is at the resonance frequency. Thus, a resonant motor can
efficiently be driven with a driving frequency of the periodic
voltage signal at or close to the resonance frequency of the
resonant motor, even though driving the resonant motor with a
periodic voltage signal having a driving frequency different to the
resonance frequency is possible as well, but leads to a less
efficient driving (more energy is needed for achieving the same
amplitude as at the resonance frequency).
The resonant motor typically comprises a motor coil and at least
one movable motor armature carrying at least one permanent magnet
assembly having at least one permanent magnet fixedly connected
with the motor armature. The motor armature is held in a rest
position by at least one spring element. The resonant motor is
driven into an oscillating motion by application of a periodic
voltage signal (see below for a discussion of the periodic voltage
signal) at the motor coil such that a current flow from a voltage
source such as a battery or an accumulator through the motor coil
is stimulated. The permanent magnet assembly of the motor armature
interacts with the electromagnetic field that is generated by the
current flowing through the motor coil. By this interaction, the
motor armature, which is held in a rest position by means of the at
least one spring element, is forced to move out of its rest
position against the spring force of the at least one spring
element. When the electromagnetic interaction ceases or changes its
direction (e.g. the periodic voltage signal changes its sign
between the two half cycles per period), the armature moves back
towards its rest position and also beyond the rest position (until
it reaches its maximum deflection amplitude) so that finally the
armature is driven into an oscillatory motion by continuous
periodic application of the voltage signal. The oscillatory motion
occurs with the driving frequency at which the periodic voltage
signal is applied at the resonant motor (i.e. the driving frequency
determines the oscillation frequency of the oscillating motion of
the resonant motor).
A "periodic voltage signal" in accordance with the present
disclosure means a voltage signal that has periodically recurring
non-zero voltage signal content to provide the external driving
force for the motor. As the frequency of the periodically recurring
voltage signals determines the oscillation frequency of the
resonant motor, the periodic voltage signal has a period that is
divided into two equally long half-cycles. In some embodiments, a
non-zero voltage signal is present in both half cycles (but with
opposite signs to drive the resonant motor in the two oscillation
directions), but it is not necessary to drive the resonant motor in
both half cycles. In some embodiments, the periodic voltage signal
has a non-zero voltage signal only in one of the two half-cycles of
each period. While the periodic voltage signal might be a
P-periodic function (P being the period) in a mathematical sense,
i.e. (f(x+P)=f(x)), this is not necessary and also often not
productive as the resonant motor may need varying driving force in
consecutive periods under different load conditions to oscillate
with an essentially constant amplitude. What remains constant is
the period of the recurrence of the voltage signals, i.e. the
length of the period and of the half cycles (or in other words: the
driving frequency), which shall not exclude that the driving
frequency can be influenced by a user as will be explained further
below. It is also not necessary that the time-integral over the
voltage signal is identical for both half cycles (i.e. the energy
fed to the resonant motor may be different in the two half cycles
of a period), as was already made clear with respect to driving the
resonant motor by applying a non-zero voltage signal in only one of
the half cycles. In some embodiments, the time integrals over the
voltage signals in the two half cycles are finite but
different.
Thus, in some embodiments the voltage signals applied at the
resonant motor in the two half cycles per period of the periodic
voltage signal have opposite signs and further in particular the
voltage signal applied in one half cycle has different sign but
same absolute voltage level than the voltage signal applied in the
other half cycle, which shall not exclude embodiments in which the
absolute voltage levels in the two half cycles are not identical.
In some embodiments, the periodic voltage signal has zero voltage
in one of the half cycles (similar to the excitation function that
a person applies at a swing, where also only energy is applied at
the swing in one movement direction). Providing a periodic voltage
signal that has zero voltage in one of its half cycles has been
found to be less energetically efficient than applying a voltage
signal in both half cycles.
For sake of clarity, the driving of a resonant motor is different
to the driving of a DC motor, where the frequency of an applied
voltage signal does not determine the rotation frequency, but where
the rotation frequency is depending on a height of a voltage
applied at the DC motor (e.g. US 2011/005015 A1 describes a DC
motor that is driven into a rotation by application of an average
voltage signal provided by a PWM signal of a certain duty
cycle--the higher the duty cycle of the PWM signal--i.e. at
constant frequency of the PWM pulses--, the higher is the rotation
frequency). In US 2011/005015 A1 two different rotation frequencies
of the DC motor are used to excite different mechanical resonant
modes of a replacement brush. The DC motor itself is not a resonant
motor.
The driving frequency in the present disclosure may typically be at
or close to the resonance frequency of the resonant motor in order
to achieve high efficiency. But obviously, this is just an
efficiency consideration and the resonant motor can virtually be
driven at any driving frequency (with reduced efficiency), which
driving frequency in turn leads to an oscillating motion of the
resonant motor having an oscillation frequency that is equal to the
driving frequency.
The moving permanent magnet assembly also induces a voltage across
the motor coil and thus a current flow through the motor coil,
which induced electric current flow typically is smaller than the
electric current flow from the voltage source. The induced voltage
is a measure for the velocity of the armature and due to the direct
relationship also for the amplitude of the armature. The previously
mentioned document WO 2004/034561 A1, which shall be incorporated
herein by reference, generally describes how a resonant motor is
driven in particular by an alternating periodic voltage signal. The
armature of the resonant motor may in particular be arranged for a
linear reciprocating movement or for an oscillating rotating
movement.
It is generally known (e.g. from document WO 2004/034561 A1) to
drive a resonant motor by applying a periodic voltage signal that
comprises only a single voltage signal of a certain duty period per
half cycle (where the length of the duty cycle may be controlled to
compensate for different load situations). I.e. if the oscillation
frequency of the resonant motor is f.sub.o (e.g. in a non-limiting
example f.sub.o is 100 Hz), than the driving frequency f.sub.d may
be set to f.sub.o, i.e. f.sub.d=f.sub.o. A full cycle of the
periodic voltage signal (and hence also of the oscillating motion
of the resonant motor) thus last 0.01 seconds and a half cycle
0.005 seconds. In this known example, one voltage pulse is provided
per half cycle, so that a pulse frequency of the periodic voltage
signal is twice as high as the driving frequency, i.e.
f.sub.p=2f.sub.d. It has now been found that a resonant motor can
be driven into a much smoother and more silent oscillating motion
if instead of a single voltage pulse per half cycle of the
oscillating motion, the applied periodic voltage signal
approximates a sine-wave voltage signal or another similar function
at least for a certain fraction of a cycle of the periodic voltage
signal.
In some embodiments discussed herein, the voltage signal (that
otherwise approximates a continuous, e.g. sinusoidal function) may
be set to zero for a certain time span in some half cycles--e.g. in
the first half cycle of each 5th period of the applied periodic
voltage signal--or the voltage may be set to zero for a certain
time span in one of the two half cycles of each period (this half
cycle may always be the first or the second half cycle or this half
cycle may alternate between the first and the second half cycles).
The time span during which the voltage signal is then set to zero
may be chosen to allow measuring the mentioned induced voltage in
the motor coil at otherwise zero external current flow in order to
achieve a parameter indicative of the velocity and amplitude of the
moving motor armature of the resonant motor and thus to allow
controlling the periodic voltage signal such that a constant
amplitude is achieved even under changing load conditions.
A personal hygiene device with a resonant motor in accordance with
the present disclosure has a motor control unit that can provide a
selectable (digitally synthesized) periodic voltage signal at the
resonant motor; in particular the periodic voltage signal may be
selected to be a sinusoidal voltage signal. An ideal sinusoidal
voltage signal does not comprise any harmonics and thus tends to
lead to a smoother operation of the overall personal hygiene device
and noise and vibrations that are caused by harmonics are
efficiently reduced. The synthesizer circuit in accordance with the
present disclosure digitally synthesizes a smooth periodic voltage
signal from a high number of voltage pulses of variable length,
where the voltage pulses are provided at a pulse frequency higher
than the driving frequency so that at least in one of the half
cycles per period two voltage pulses are provided (hence, the pulse
frequency is then at least four times higher than the driving
frequency). The pulse frequency is determined by the constant
(temporal) distance between the voltage pulses. In some
embodiments, the pulse frequency is at least six times higher than
the driving frequency (i.e. the voltage signal in each half cycle
is approximated by at least three voltage pulses), optionally at
least 20 times higher (at least 10 pulses per half cycle) and
further optionally at least a 100 times higher (at least 50 pulses
per half cycle) than the driving frequency. While the voltage
signal as generated comprises individual pulses, the
characteristics (e.g. capacitance and inductance) of the motor
filter the pulses such that the motor "sees" a continuous voltage
signal. Even though a digitally synthesized sinusoidal voltage
signal as described does not necessarily result in an ideal
sinusoidal signal, it had been found that noise reductions of up to
-10 dB can be achieved between driving a resonant motor of a
personal hygiene device with a rectangular driving function (i.e. a
single rectangular voltage pulse applied per half cycle of the
periodic voltage signal) and with an almost sinusoidal voltage
signal that is digitally synthesized as herein described. A
sinusoidal voltage signal also leads to a sinusoidal current flow
through the motor coil. It shall be understood that the
approximation quality of a digitally synthesized periodic voltage
signal (details of the voltage signal synthesis are described
further below) vs. an ideal sinusoidal voltage signal depends on,
e.g., the pulse frequency to driving frequency ratio and thus also
only an approximate sinusoidal current results. During each voltage
pulse, a current flow into the coil builds up and if the voltage
pulse is interrupted until the next voltage pulse is provided, then
the charge stored in the coil flows out of the coil to a certain
extend.
A personal hygiene device in accordance with the present disclosure
may be an electric toothbrush, an electric tongue cleaner, an
electric flossing device, an electric shaver, an electric hair
removal device, an electric skin massaging device or the like.
FIG. 1 is a schematic depiction of a personal hygiene device 1 in
accordance with the present description. The personal hygiene
device 1 is here realized as an electric toothbrush, which shall
not be considered as limiting. The personal hygiene device 1
generally comprises a head section 10 that is driven into an
oscillatory motion (either the whole head section 10 is driven into
such an oscillatory motion or the head section 10 comprises a head
element 11 that is driven into the oscillatory motion) by a
resonant motor (see FIG. 2) provided in a handle section 20 of the
personal hygiene device 1. The personal hygiene device 1 may have
an on/off switch 21 and optionally a mode selector button 22, even
though the personal hygiene device 1 may not necessarily need to
have these features (e.g. the personal hygiene device 1 may be
arranged to automatically switch on the resonant motor if the head
11 is close to tissue, which may be detected by a capacitive
threshold sensor and/or the personal hygiene 1 has no switchable
modes or a mode selection may be implemented in another manner,
e.g. via voice recognition).
In some embodiments, the personal hygiene device 1 has a user
controllable input unit 30 for providing user-selected input
influencing the periodic voltage signal applied at the resonant
motor via a motor control unit 50 as will be explained in more
detail further below. Generally, the user may be able to influence
the shape of the periodic voltage signal or the frequency of the
periodic voltage signal, the frequency of the pulses used to
approximate the ideal periodic voltage signal (see below) etc. In
some embodiments, the user controllable input unit 30 has a control
element 31 via which a user can selectively influence the periodic
voltage signal applied at the resonant motor via the motor control
unit. Additionally or alternatively, the user controllable input
unit 30 may comprise a separate control device 40 (i.e. a separate
control device physically separate from the handle section 20). The
personal hygiene device may then comprise a wireless connection
unit 33 for establishing a wireless connection 34 between the
separate control device 40 and the handle section 20 so that e.g.
data can be communicated in a wireless fashion from the separate
control device 40 to the handle section 20 and thus to the motor
control unit 50. The wireless connection 34 may in particular be
realized as a Bluetooth connection, but other wireless connection
standards are as well possible, e.g. an IEEE 802.11 radio frequency
connection or a proprietary wireless connection. Generally, the
separate control device 40 comprises a control element 42 via which
the user can influence the periodic voltage signal used for driving
the resonant motor. The control element 42 may be realized as a
switch or selector button, a slider or the like. In some
embodiments, the separate control device 40 comprises a
touch-sensitive screen 41 on which a virtual control element 42 can
be displayed, which can be tuned by touching the screen 41 with a
finger and sliding the finger over the screen. In the shown
example, the virtual control element 42 is realized as a virtual
slider by which the user can influence the periodic voltage signal
to be applied at the resonant motor, e.g., the user can set whether
the periodic voltage signal has a sinusoidal shape or a rectangular
shape and potentially the user can set at least one further shape
of the periodic voltage signal having a more intermediate character
between a sinusoidal shape and a rectangular shape. In some
embodiments, the separate control device 40 is realized by a smart
phone, by a tablet computer or any other mobile appliance. The
separate control device 40 may then have a software module (such as
a mobile application software or "app") provided for realizing the
virtual control element 42 and for transmitting the setting chosen
by a user from the separate control device 40 to a receiver 32 in
the handle section 20. As the shape of the periodic voltage signal
tends to influence the noise characteristic of the personal hygiene
device 1 during operation, such a user controllable input device 30
as described allows a user to set a personally favored periodic
voltage signal, e.g. a periodic voltage signal that generates less
(or more) noise (or sound) than the standard periodic voltage
signal set by the manufacturer of the personal hygiene device 1.
E.g. the manufacturer may have chosen a periodic voltage signal at
which the energy consumption of the resonant motor is relatively
low but where the noise or sound level of the personal hygiene
device is at a medium level or where the noise or sound of the
personal hygiene device is perceived by an individual user as less
favorable due to spectral components in the noise or sound than the
noise or sound generated with a different periodic voltage signal.
Some users may favor less noise as they get annoyed by the noise,
while other users may favor more noise as they connect the sound of
the personal hygiene device with its hygienic properties (e.g. in
case of an electric toothbrush, a high sound level may be assigned
to a high presumed cleaning power). The influencing possibilities
described above with respect to a separate control device can also
be applied in case of a user controllable input unit that is not
separate and is, e.g., realized as a part of the handle section of
the personal hygiene device.
FIG. 2 is a schematic depiction of a motor control unit 100 for
driving a resonant motor 200 (which may be disposed in a handle
section of the personal hygiene device as was mentioned before)
into an oscillating motion, e.g. a linear reciprocating motion or
an oscillating rotation or a mixture thereof. The resonant motor
200 is arranged in the bridge section of an H-bridge (or: full
bridge) circuit comprising four switches 191, 192, 193, and 194.
The switches of the H-bridge circuit are controlled by a switch
control unit 110 and, as has been discussed in previously mentioned
document WO 2004/034561 A1, a voltage supplied from a voltage
source 210 can then be applied in a positive direction by switching
on switches 191 and 194 and switching off switches 192 and 193 and
in a negative direction by switching on switches 192 and 193 and by
switching off switches 191 and 194. It is as well possible to
short-circuit the resonant motor 200 by, e.g., switching on
switches 193 and 194 and switching off switches 191 and 192 (again,
as is described in document WO 2004/034561 A1). The switches 191 to
194 may each be realized by a field effect transistor (FET), in
particular by a MOSFET. The switches 191 to 194 may in particular
each comprise an in parallel connected protection diode for
protecting the respective switch from overvoltage. The switches 191
to 194 are also chosen such that they can be switched with the
pulse frequency required by the motor control unit 100, e.g. 30 kHz
as one non-limiting example.
While document WO 2004/034561 A1 describes that a single voltage
pulse is applied at the resonant motor in each half cycle of each
period, the herein proposed motor control unit 100 comprises a
synthesizer circuit for providing voltage pulses of varying pulse
length at the resonant motor at a pulse frequency that is at least
four times higher than the driving frequency at which the resonant
motor is driven. The idea behind the application of voltage pulses
at a respectively high pulse frequency is to model a target shape
of the average periodic voltage signal by the voltage pulses having
essentially constant height (the voltage height may be determined
by a voltage source) but varying pulse length (digital synthesis).
An (ideal) sinusoidal periodic voltage signal would then lead to a
sinusoidal current flow through the motor coil, as had previously
been explained. Typically, a resonant motor in a personal hygiene
device may be driven at a driving frequency of between about 50 Hz
to about 500 Hz, which shall not exclude other driving frequency
values. Electric toothbrushes are often driven at a frequency of
between about 65 Hz to about 300 Hz. As a non-limiting example, a
driving frequency of 150 Hz may be used. The pulse frequency is
given by the constant temporal distance between consecutive voltage
pulses; the pulses may have varying pulse length in order to model
the target shape of the periodic voltage signal. The pulse
frequency should be at least four times higher than the driving
frequency, in particular the pulse frequency is at least 6 times
higher than the driving frequency (at least three voltage pulses
are then applied per half cycle), optionally the pulse frequency is
at least 20 times higher than the driving frequency (at least ten
voltage pulses are then applied per half cycle) and further
optionally the pulse frequency is at least a hundred times higher
than the driving frequency (at least 50 voltage pulses are then
applied per half cycle). E.g. at a driving frequency of 150 Hz, the
pulse frequency may then be at least 900 Hz, at least 3 kHz, or at
least 15 kHz. Generally, the pulse frequency may be above 18 kHz
and optionally above 20 kHz in order to shift the pulse frequency
into a non-audible (for the human ear) frequency range. The pulse
frequency may be chosen to be below 100 kHz.
In accordance with the present disclosure, the motor control unit
provides via its synthesizer circuit voltage pulses of variable
length at the resonant motor. In order to allow a sensible shaping
of the average periodic voltage signal, the length of each voltage
pulse should be controllable with sufficient resolution, which
requires that the voltage pulse length can be controlled at a
clocking frequency of the motor control unit that is higher than
the pulse frequency, e.g. 128 times higher (resulting in a 7 bit
resolution of the voltage pulse) or 256 times higher (8 bit
resolution) (even though higher or lower resolutions such as 9 bit
or 10 bit or 6 bit or 5 bit or 4 bit etc. shall not be excluded).
E.g. at 15 kHz pulse frequency, the clocking frequency would be
3.84 MHz for an 8 bit resolution. As another example, the driving
frequency is 150 Hz, the pulse frequency is 30 kHz and the
resolution is 7 bit (again leading to a clocking frequency of 3.84
MHz).
FIG. 3 is a schematic depiction of an example motor control unit
1000 having an example synthesizer circuit 120 in accordance with
the present disclosure. The synthesizer circuit 120 as shown
comprises a switch control unit 121 for switching switches 191 to
194 of an H-bridge as shown in FIG. 2, a clock 122 for proving a
clocking frequency (e.g. 3.84 MHz) and a memory unit 123. The
memory unit 123 may in particular comprise at least one look-up
table of normalized voltage pulse length values to be applied
during one half cycle or during one period of the periodic voltage
signal. In case that the voltage signal applied during the second
half-cycle is identical but inverted to the voltage signal applied
during the first half cycle of each period, then it is sufficient
to just provide the voltage pulse length values for the first
half-cycle (the switches of the H-bridge are used to invert the
sign of the voltage applied at the resonant motor).
In some embodiments, the memory unit 123 comprises at least two
look-up tables of voltage pulse length values, e.g. one look-up
table for a sinusoidal periodic voltage signal and one look-up
table for a rectangular periodic voltage signal. In some
embodiments, three or more look-up tables are provided, where e.g.
the third look-up table provides voltage pulse length values for a
periodic voltage signal resembling an intermediate shape between a
sinusoidal and a rectangular shape. In some embodiments, two, three
or more such as five or ten etc. look-up tables are provided for
intermediate periodic voltage signal shapes such that a user could
finely tune (via the previously described user controllable input
device) the shape of the periodic voltage signal to lie between a
sinusoidal and a rectangular shape. The synthesizer circuit 120 may
therefore be arranged to receive an input signal 124 from the user
controllable input device 30 discussed with reference to FIG. 1. In
some embodiments, at least one look-up table is provided for
generating a periodic voltage signal different to a sinusoidal or
rectangular shape (or an intermediate shape between those two),
e.g. for generating a periodic triangle signal, a periodic trapeze
signal, or a periodic saw tooth signal, even this list shall not be
considered as closed and any other periodic voltage signal shape
may be employed as well. If a separate control device as discussed
with reference to FIG. 1 is used, the respective application
software module may be arranged to allow the user to freely define
an arbitrary periodic voltage signal shape. The synthesizer circuit
120 may be realized as a direct digital synthesis (DDS) circuit
(e.g. the user may be allowed to draw the shape with a finger
gliding over a touch sensitive display). As a non-limiting example,
at least part of the synthesizer circuit 120 may be realized by the
low power DDS AD9838 chip (or a similar IC) available from Analog
Devices, Norwood, Mass., USA. In other embodiments, the synthesizer
circuit is realized (optionally together with the switches of the
H-bridge) as an integrated circuit (IC), in particular an
application specific IC (ASIC). In addition or alternatively, the
synthesizer circuit may comprise a computing unit that computes the
voltage pulse length values for, e.g., a sinusoidal voltage
function in real time instead of using a look-up table.
In some embodiments and indicated in FIG. 3 with dashed lines, the
motor control unit 1000 additionally comprises a digital voltage
circuit 160 that is arranged for providing a single rectangular
voltage pulse per half cycle at the resonant motor as is known from
prior art. A voltage generation control circuit 180 may be provided
for selectively switching on either the synthesizer circuit 120 or
the digital voltage circuit 160. Both, the synthesizer circuit 120
and the digital voltage circuit may thus be coupled with switches
191, 192, 193, 194 of an H-bridge, and the voltage generation
control circuit 180 would selectively allow only one of these two
circuits 120, 160 to control the switches. In some embodiments, the
synthesizer circuit 120 may be used to provide a first part of a
periodic voltage signal by a plurality of short voltage pulses
(e.g. an upwards voltage ramp) and then the voltage generation
control circuit 180 switches to the digital voltage circuit 160 to
generate a single long voltage pulse as a second part of the
periodic voltage signal per half cycle. Optionally, a third part of
the voltage signal may then again be applied by the synthesizer
circuit 120, e.g. a downwards voltage ramp so that, e.g., a trapeze
signal is generated together with the upwards ramp and the long
voltage pulse. Obviously, long rectangular voltage signals can as
well be shaped by a synthesizer circuit instead of an analog
voltage circuit. It should also be understood that instead of
applying voltage pulses via switching the switches 191 to 194 of
the H-bridge (see FIG. 2), a periodic voltage signal generated by a
synthesizer circuit can be directly applied at the resonant motor
(the synthesizer circuit would then comprise the necessary switches
for switching the voltage pulses from which the periodic voltage
signal is synthesized).
FIG. 4 is a schematic depiction of an example sinusoidal periodic
voltage signal generated from a plurality of short voltage pulses
of variable length but constant height, where only the first
half-cycle of a period of the approximate sinusoidal periodic
voltage signal applied at the resonant motor is shown. It is
understood that the second half-cycle may have the same functional
behavior but with an inverted voltage sign. In FIG. 4, the first
half-cycle of the sinusoidal periodic voltage signal is exemplary
generated by applying 10 voltage pulses 301 to 310 (i.e. the pulse
frequency is 20 times the driving frequency, e.g. at 150 Hz driving
frequency this leads to a 3 kHz pulse frequency). As had been
explained above, the voltage pulse length values for each of the
voltage pulses 301 to 310 may be provided as tabularized values in
a memory unit and may have been predetermined so that in average an
approximate sinusoidal voltage results. FIG. 4 comprises a
magnification of the third voltage pulse 303 and it is indicated by
sixteen (16) tick marks 400 that the resolution in the shown case
is four (4) bit (this is a non-limiting example and was also chosen
for presentability of the general concept), so that a clocking
frequency of 48 kHz is needed in this example case. In the
schematic depiction, the third voltage pulse 303 has a pulse length
W3 of seven clocking frequency periods and then a voltage off
length O3 of nine clocking frequency periods follows (until the
fourth voltage pulse 304 is switched on). As is also indicated in
FIG. 4, the maximum voltage V.sub.max provided at the resonant
motor may be lower than the available voltage V.sub.B from the
voltage source (e.g. V.sub.max could be 60% of V.sub.B). This
allows increasing the voltage level at the resonant motor under a
load condition when the resonant motor requires more energy to
provide the same oscillatory amplitude (e.g. the tabularized
voltage length values may then be increased by a conversion factor
>1 reflecting the load state).
As has been explained in the previous paragraph, a load applied at
the resonant motor may lead to reduced motion amplitude if the
energy provision is not adequately adapted. The motor load of a
resonant motor can be determined by determining the back EMF
voltage of the motor (i.e. the voltage that is induced in the motor
coil by the moving permanent magnet assembly of the moving
armature) as the induced voltage is a measure of the velocity of
the armature (which in turn is a measure of the amplitude of the
armature movement as the oscillation frequency of course stays
constant under varying load as it is given by the driving
frequency). One method to determine this induced voltage is to
provide a further coil positioned close to the armature, which
involves further costs and further parts. Another method is to
measure the back EMF at the motor coil when essentially no motor
current flows (as then the applied voltage as well as the
self-induced voltage are essentially reduced to zero). But if a
sinusoidal periodic voltage signal is provided as driving signal at
the motor coil, a sinusoidal current results and thus there is no
time slot during the period at which no current flows through the
motor coil. In some embodiments it is thus proposed to switch off
the sinusoidal or any other continuous (or semi-continuous) driving
signal at least during one of the half cycles of each period or of
each 5.sup.th or 10.sup.th etc. period at least for a time period
that allows the motor current to drop to zero and to stay at zero
until a measurement of the back EMF has been made. The motor may be
short circuited to achieve a fast current discharge. In some
embodiments, the provision of voltage pulses is resumed in the same
half cycle in which the provision of voltage pulses was stopped
after the measurement of the back EMF was made. This may lead to
the generation of harmonics due to switching on a voltage of a
relatively high value after having provided a zero voltage. In some
embodiments, the voltage pulse provision is switched off in the
complete second quadrant of the half cycle in which the voltage is
switched off. It had been found that this represents a good balance
between current consumption and noise generation on the one hand
and reliability of the back EMF measurement on the other hand.
Due to manufacturing tolerances, a resonant motor may not always
have the same resonance frequency, which may be determined at the
end of assembling the resonant motor by the manufacturer. In some
embodiments, it may be considered important to always have the same
difference between the resonance frequency of the resonant motor
and the driving frequency applied by the motor control unit, it may
become necessary to apply a different driving frequency than
originally planned. E.g. a driving frequency of 150 Hz may have
been planned and respectively 100 voltage pulse length values had
been provided for a half cycle in the memory unit of the
synthesizer circuit. But due to differences in the resonance
frequency of the resonant motor, the driving frequency may need to
lie in a range of between about 145 Hz to about 155 Hz. In the
given example, a single voltage pulse relates to about 0.75 Hz so
that in case that a reduced driving frequency of 145 Hz is to be
employed, 103.45 pulses need to be employed per half-cycle (it is
assumed that the clocking frequency as well as the pulse frequency
are fixed values). In order to cope with this situation, the
driving frequency may e.g. be set to about 144.75 Hz and 7 voltage
pulse length values could be employed twice per period (in case of
a required higher driving frequency, some voltage pulses may be
omitted). This allows using the available look-up tables also for
other frequencies. In some embodiments, the user may be allowed to
influence the driving frequency via a user controllable input
unit.
The dimensions and values disclosed herein are not to be understood
as being strictly limited to the exact numerical values recited.
Instead, unless otherwise specified, each such dimension is
intended to mean both the recited value and a functionally
equivalent range surrounding that value. For example, a dimension
disclosed as "40 mm" is intended to mean "about 40 mm."
Every document cited herein, including any cross referenced or
related patent or application and any patent application or patent
to which this application claims priority or benefit thereof, is
hereby incorporated herein by reference in its entirety unless
expressly excluded or otherwise limited. The citation of any
document is not an admission that it is prior art with respect to
any invention disclosed or claimed herein or that it alone, or in
any combination with any other reference or references, teaches,
suggests or discloses any such invention. Further, to the extent
that any meaning or definition of a term in this document conflicts
with any meaning or definition of the same term in a document
incorporated by reference, the meaning or definition assigned to
that term in this document shall govern.
While particular embodiments of the present invention have been
illustrated and described, it would be obvious to those skilled in
the art that various other changes and modifications can be made
without departing from the spirit and scope of the invention. It is
therefore intended to cover in the appended claims all such changes
and modifications that are within the scope of this invention.
* * * * *